The present invention relates to a high-frequency, high-resolution cardiac biopotential analysis apparatus and method, and more particularly to a microcomputer-based cardiac biopotential analysis apparatus for qualitatively determining in a noninvasive manner, cardiac phenomena that can be ascertained by analyzing cardiac electrical activity.
Cardiac biopotentials arise from the discharge of hundreds of thousands of electrically active cells. The signal detected at the body surface is a composite determined by different types of tissue, differing locations of that tissue, and the type of organization (or disorganization) of the wavefront of activation. When transmitted to the body surface the signal is altered in morphology and frequency content as a result of such factors as body fat, rib cage size, and position of the heart in relation to the lungs. All these variables lead to challenging signal processing problems.
Despite nearly a century of use, the conventional scalar electrocardiogram ("ECG") has major shortcomings. Its value for the diagnosis of coronary artery disease (CAD) is limited. It is very useful when there has been an antecedent myocardial infarct (MI) ("heat attack") which leads to localized fibrosis extensive enough to be detectable. In the absence of previous MI, the abnormalities induced by CAD in the resting ECG in an asymptomatic individual are of limited sensitivity and specificity.
The scalar ECG is of much greater value for the detection of active, ongoing ischemia. Monitoring of the ST segment during chest pain is a reliable indicator of cardiac ischemia and is used diagnostically when chest pain spontaneously presents itself or when chest pain is deliberately provoked for diagnostic purposes as in exercise stress testing. However even for this diagnostic application there occur significant numbers of false positives often requiring further, more expensive noninvasive tests (nuclear imaging) or invasive assessment through the use of cardiac catheterization and coronary angiography.
For the assessment of the risk of sudden cardiac death due to malignant ventricular arrhythmias the conventional ECG is of practically no use whatsoever. Twenty four hour continuous ambulatory monitoring of the scalar ECG ("Holter monitoring") is of some value in the minority of individuals with significant amounts of ventricular extrasystoles, but at considerable expense and inconvenience to the patient.
Advances in computer technology have led to attempts to improve the diagnostic information extracted from the surface ECG. One such approach is the cardiointegram (CIG) which has been used for the detection of coronary disease. This approach, as described in "The Cardiointegram: Detection of Coronary Artery Disease in Males with Chest Pain and a Normal Resting Electrocardiogram", J. Electrocardiography. 19(3): pp. 257-267 (1986), applies a process of integration over the various sections of the QRST signal thereby highlighting information about the interrelationships of positive to negative deflections from the ORS to the TO interval and T wave amplitude. Using this technique it has been shown that coronary artery disease can be detected from the resting, normal ECG with a sensitivity and specificity slightly less than exercise stress electrocardiography.
More recently Abboud et. al in "High Frequency Electrocardiography Using an Advanced Method of Signal Averaging for Non-Invasive Detection of Coronary Artery Disease in Patient with Normal Conventional Electrocardiogram", Electrocardiography. 19(4): pp. 371-380 (1986) showed that high frequency components of the ECG (150-250 Hz) averaged in the frequency domain after the fast-Fourier transform exhibited a characteristic "zone of reduced amplitude" in patients with CAD. The sensitivity of this technique was 75%. CIG had similar sensitivity.
A second area in which new computer based techniques have been applied to electrocardiography is in the detection of patients at risk for malignant ventricular arrhythmias and sudden cardiac death. Simson in "Use of Signals in the Terminal QRS Complex to Identify Patients with Ventricular Tachycardia After Myocardian Infarction", Circulation. 64(2): pp. 235-242 (1981) showed that signal averaging in the time domain reveals the presence of low amplitude high frequency deflections in the terminal portion of the QRS complex, so-called 'late potentials". These late potentials have been correlated with inducibility of serious arrhythmias in the electrophysiology (EP) lab and with an increased risk of sudden death during longterm follow up of survivors of heart attack. However, the detection of late potentials has a poor predictive accuracy due to the problem of false positive tests.
An alternative approach to the detection of risk for arrhythmia uses indices of the power spectrum of the signal averages QRS. Cain et al in "Fast-Fourier Transform Analysis of Signal-Averaged Electrocardiograms for Identification of Patients Prone to Sustained Ventricular Tachycardia", Circulation. 69(4): pp. 711-720 (1984) showed that this approach can distinguish arrhythmia patients from controls and correlates with inducibility of arrhythmias in the EP lab. Recent attempts to reproduce such results have met with varied success, due to fundamental problems in defining length of segment for FFT and in distinguishing the end of the QRS from noise. Haberl et. al in "Comparison of Frequency and Time Domain Analysis of Signal Averaged Electrocardiogram in Patients with Ventricular Tachycardia and Coronary Artery Disease: Methodologic Validation and Clinical Relevance. JACC. 12(1): pp. 150-158 (1988) applied successive FFTs that are shifted in time to the terminal portion of the signal averaged QRS to address some of the problems with the Cain method. Neither the time-domain indices of "late potentials" nor the power spectral indices of Cain or Haberl have been shown to be influenced by drugs. This has limited the application of these technologies to diagnostics, where the problem of false positives leads to their use as additional procedures at additional cost. Judgement of therapeutic efficacy continues to require additional costly invasive and noninvasive procedures.
The fundamental limitation of techniques applied to the ECG to date is their linear nature. The cardiac electrical signal is a complex summary of spatial and temporal inputs and many nonlinear dynamic features should be expected. In particular, neural inputs to the heart will have significant nonlinearities. What is true in health is at least equally true in disease. Thus a disease process can be expected to lead to characteristic alterations in nonlinear properties as well as linear ones. An ability to quantity abnormalities in nonlinear dynamics would therefore be expected to enhance diagnostic power and improve the assessment of therapeutic efficacy.
It is therefore a principal object of the present invention to provide a noninvasive system and method for reliably determining myocardian physiologic properties.
Another object of the present invention is to provide a noninvasive system and method for quantifying linear and nonlinear properties of phase and energy components within the frequency structure of the electrocardiogram.
A further object of the present invention is to provide a noninvasive system and method for diagnosing and quantifying coronary artery disease.
Another object of the present invention is to provide a noninvasive system and method for the detection and quantification of myocardial ischemia in real time, for example as a part of intraoperative monitoring.
Another object of the present invention is to provide a noninvasive system and method for the detection of successful reperfusion of the infarct-related artery in patients given thrombolytic therapy for acute myocardian infarction.
A further object of the present invention is to provide a noninvasive system and method for the assessment of coronary artery restenosis after successful percutaneous transluminal coronary angioplasty.
A further object of the present invention is to provide a noninvasive system and method for the quantification of cardiac electrical stability fixed or real time and the propensity for arrhythmias whether due to drugs, heart disease, or neural factors.
A further object of the present invention is to provide a noninvasive system and method for the quantification of the extent of malignancy of cardiac arrhythmias.
A further object of the present invention is to provide a noninvasive system and method for the identification of wide-complex supraventricular tachycardia from sustained vertricular tachycardia.
A further object of the present invention is to provide a noninvasive system and method for assessing the efficacy of therapy for arrhythmias and sudden cardiac death whether that therapy is drugs or surgery.
A further object of the present invention is to provide a noninvasive system and method for quantifying the effects of neural and humoral inputs to the heart, including the sympathetic and parasympathetic systems.
A further object of the present invention is to provide a noninvasive system and method for evaluating pump function and quantifying ejection fraction.
It is still another object of the present invention to provide a noninvasive system and method for quantifying the effects of ongoing organ rejection in cardiac transplant patients.